The present invention relates to nonformaldehyde, nonfuming resorcinolic resins, and methods of making and using the same. These resins are particularly useful when combined with curing agents in rubber, imparting improved physical and mechanical properties such as low volatility, reduced fuming in rubber compounding, and improved adhesion properties of vulcanized rubber and rubber composites.
Resorcinol and resorcinol-formaldehyde resins have been used in the rubber industry as reinforcing and bonding agents in rubber compounds. These resins are unique materials for rubber compounding, since they act as thermosetting and vulcanizing plasticizers. They are very efficient plasticizers for rubber during the processing operations. Use of these resins allows easier processing, higher loading and excellent extrusions for the rubber compounds.
The thermosetting properties of resorcinol upon curing provide the vulcanizate with increased hardness, abrasion resistance, aging resistance, solvent and oil resistance, and stiffness as compared with vulcanizates made without resorcinol or its derivatives; resorcinol also gives much improved finishes to the cured rubber stock. This combination of plasticizing and reinforcing action is rare for a single material in rubber compounds.
Although resorcinol imparts good mechanical and adhesion properties to rubber, fuming of resorcinol during compounding at a temperature in excess of 110xc2x0 C. can occur. To overcome the fuming problems of resorcinol, tire manufacturers are seeking modified resorcinolic derivatives and resins that do not produce volatiles at Banbury temperatures. In order to reduce fuming completely, the compounds used should not contain any free resorcinol, or should have levels of free resorcinol of about 1.0 wt. % or less.
One of the ways to reduce the fuming of resorcinol in the rubber compound is to use resorcinol-formaldehyde resins (RF resins) in place of resorcinol. Generally, RF resins are produced by the reaction of resorcinol with formaldehyde in the presence of an acid catalyst. The free resorcinol content of RF resins can be reduced by increasing the formaldehyde level. When the formaldehyde content is increased in the RF novolak synthesis, however, the softening point of the final material is also increased due to an increase in its molecular weight. If the formaldehyde level is increased beyond a certain level, the final resin will become a gel. Due to these limitations, it is very difficult to develop an RF type resin containing less than 1.0 wt. % free resorcinol with a softening point less than 105xc2x0 C. For example, U.S. Pat. Nos. 2,746,898 and 3,596,696 disclose the preparation of RF resins, but the free resorcinol content in these resins is well above 1 wt. %, and, hence, the fuming problems exist with these resins. In addition, RF resins tend to absorb moisture upon exposure to humidity, and therefore soften and coalesce during storage.
Another approach developed to address the problem of resorcinol fuming is the use of derivatives of resorcinol such as alkyl, aralkyl, monoester and monoether compounds. Simple alkyl substituted resorcinol, such as methylresorcinol, is difficult to synthesize. Aralkyl substituted resorcinols often have low melting points, and are paste-like and difficult to handle by the tire industry. Synthesis of these derivatives often requires extensive processing steps that involve different organic solvents and isolation procedures. In addition, if organic solvents are used, their handling and disposal often bring more problems and cost.
U.S. Pat. No. 4,605,696 discloses the synthesis of resorcinol monobenzoate and resorcinol monorosinate and their use in the rubber compound formulation. The synthesis of monorosinate involves the use of a xylene solvent, requiring distillation and disposal of solvent waste. Resorcinol monobenzoate has a higher melting point than resorcinol and, therefore, a processing problem exists in the rubber compound application.
U.S. Pat. No. 4,892,908 discloses a keto derivative of resorcinol, namely 4-benzoylresorcinol, which can be used as a low-fuming resorcinolic derivative replacing resorcinol in rubber compounds. The preparation of benzoylresorcinol, however, requires the use of toxic chemicals, such as benzotrichloride, and highly volatile organic solvents. Therefore, the cost of benzoylresorcinol is about 3-4 times higher than that of resorcinol. In addition, benzoylresorcinol has a higher melting point than resorcinol and also exhibits processing difficulties.
Yet another approach to eliminate or minimize the resorcinol fuming is to alkylate or aralkylate part of the resorcinol and then react the product with formaldehyde to develop an alkyl or aralkyl substituted resorcinol-formaldehyde type resin. U.S. Pat. No. 4,889,891 discloses such resins formed by reacting an alkylsubstituted resorcinol, prepared from the reaction of resorcinol with dicyclopentadiene, dipentene, piperylene, or another composition, with formaldehyde. While the softening points of these resins are acceptable for rubber compounding, the resins still contain free resorcinol in amounts greater than 1 wt. %. U.S. Pat. Nos. 5,021,522 and 5,049,641 disclose resorcinolic resins prepared by reacting an aralkyl substituted resorcinol, prepared from the reaction of styrene and resorcinol, with an aqueous formaldehyde solution. Though the softening points of these resins are acceptable for the rubber compounds, the free resorcinol content is typically greater than 2.5 wt. %.
Several other attempts have been made to develop low-fuming or non-funming resorcinol modified resins for the tire industry. For example, resorcinol was used with phenol or alkylphenol and formaldehyde to develop phenol-resorcinol-formaldehyde (PRF) and alkylphenol-resorcinol-formaldehyde type resins. Problems with these compounds arise due to the handling of formaldehyde and solvents to effect the synthesis. Moreover, to achieve a modified resorcinolic resin containing less than 1.0 wt. % free resorcinol and having a softening point less than 105xc2x0 C., the final distillation to remove unreacted monomers must be done at temperatures in excess of 180-190xc2x0 C. and vacuum conditions of 5-6 mm of Hg. Without hot oil heating of the reactor and an efficient high vacuum pump, which require higher capital expenses, low free resorcinol levels in these resins are difficult to achieve.
U.S. Pat. No. 5,244,725 discloses the synthesis of a nonformaldehyde type resorcinolic resin from the reaction of resorcinol with bisphenol-A epoxy. Although the resin of this patent shows good dynamic mechanical properties compared to resorcinol, fuming associated with a 10.0 wt. % free resorcinol content of this resin restricts its use in the rubber compound formulations.
EP 798 324 (abstract) reports preparation of resins by reacting aromatic compounds with nonconjugated dienes in the presence of an acid catalyst. Free resorcinol content of the product, however, is still at least about two.
The present invention provides resorcinolic resins that have a wide variety of desirable properties. The resins have about 1 or less than 1 wt. % free resorcinol, and have softening points between about 75 and 1 10xc2x0 C. The resins are non-fuming and are less hygroscopic as compared to other resorcinol derivatized resins, such as RF resins. The present resins are capable of undergoing cross-linking with curing agents during rubber vulcanization to give improved physical, mechanical and adhesion properties to the rubber.
Another embodiment of the present invention provides a method for making such resins, by reacting resorcinol with dicyclopentadiene, and then further reacting that product with an olefinic compound. Significantly, the methods can be carried out at processing temperatures below 180-190xc2x0 C. Moreover, the process is carried out in the absence of both solvents and formaldehyde, thus eliminating handling hazards of waste solvents and distillates.
Yet another embodiment of this invention provides a rubber composition comprising: a) a rubber component; b) a methylene donor; and c) a methylene acceptor; the methylene acceptor is the novel resin of the present invention. Reinforced rubber articles further comprising d) a reinforcing material are also provided by the present invention.
It is therefore an aspect of the present invention to provide a resorcinolic resin that is non-fuming. s Another aspect of the invention is to provide a method for synthesizing a nonformaldehyde, non-fuming resorcinolic resin.
It is a further aspect of the present invention to provide a vulcanizable rubber composition having improved physical and mechanical properties.
It is another aspect of the invention to provide a vulcanized rubber composition having improved adhesion between the rubber and reinforcing material.
These and other aspects of the invention will be more fully understood from the following description of the invention and the claims appended hereto.
The present invention is generally directed to a method for synthesizing a nonformaldehyde, non-fuming resorcinolic resin. The methods generally comprise the steps of mixing a catalyst with melted resorcinol; adding dicyclopentadiene to the mixture; and then adding a second quantity of catalyst to the mixture followed by an olefinic compound. The resin that results from this procedure is also within the scope of the present invention. Such resins are characterized as having low free resorcinol levels. When utilized in rubber compounding applications, the present resins impart improved physical, mechanical and adhesive properties to the rubber.
As used herein, xe2x80x9clow free resorcinolxe2x80x9d refers to resins that have a low level of unreacted resorcinol. Preferably, the free resorcinol content of these resins is about 1 wt. % or less unreacted resorcinol. xe2x80x9cNon-fumingxe2x80x9d resins are therefore resins having a low free resorcinol content; because of the low levels of free resorcinol, the resins exhibit little or no fuming during processing of rubber compounds. The term xe2x80x9cnon-fumingxe2x80x9d is therefore intended to encompass any level of fuming that might result when using compounds having about 1 wt. % or less of free resorcinol. This feature is also referred to as xe2x80x9cnonvolatile.xe2x80x9d
The present methods generally involve melting resorcinol, which is effected at a temperature of between 125 and 135xc2x0 C. Following melting of the resorcinol, a catalyst is added, with stirring; the mixture should be stirred for a sufficient time to allow thorough mixing of the catalyst and the resorcinol, typically at least about two minutes. Dicyclopentadiene is then added to the resorcinol/catalyst mixture. Preferably, the dicyclopentadiene is added over a period of between about 30 minutes and two hours, more preferably a period of between 60 and 90 minutes; the temperature during addition of dicyclopentadiene should be maintained between about 125 and 155xc2x0 C., more preferably between about 135 and 145xc2x0 C. In a preferred embodiment, after addition of the dicyclopentadiene, the reaction is continued at a temperature of between about 130 to 155xc2x0 C., more preferably between about 140 and 145xc2x0 C.; the reaction is maintained at this temperature for enough time to effect reaction between the dicyclopentadiene and resorcinol, typically between about and 30 and 90 minutes, preferably about 60 minutes. Following this time, an additional charge of catalyst is added, preferably all at once. An olefinic compound is then added slowly to the mixture. As with the dicyclopentadiene, the olefinic compound is preferably added over a period of between about 30 minutes to two hours, more preferably between about 60 and 90 minutes. The temperature during addition of the olefm is preferably maintained between approximately 130 and 155xc2x0 C., more preferably between about 140 and 145xc2x0 C. Following addition of the olefinic compound, the reaction can be maintained for between about 30 and 90 minutes, preferably about 60 minutes. The mixture should then be heated to between about 150 and 165xc2x0 C., preferably about 155 to 160xc2x0 C. Stirring is maintained at this elevated temperature for at least about 30 minutes, preferably 60 minutes or longer, after which the reaction temperature can be lowered and a basic compound added to neutralize the catalyst. To remove any unreacted dicyclopentadiene or olefmic compound, vacuum distillation is preferably used, although other means known to those skilled in the art could also be employed. Such distillation can be applied prior to the second addition of catalyst, at the end of the procedure, or both.
Any acid catalyst can be used in each of the catalyst addition steps according to the present invention. Suitable acid catalysts include, but are not limited to, H2SO4, H3PO4, aromatic and aliphatic sulfonic acids, and the like. The preferred catalyst is p-toluenesulfonic acid (PTSA).
Both aromatic and aliphatic olefinic compounds can be used according to the present invention. The olefin reacts with both the resorcinol/dicyclopentadiene end product and the free resorcinol. The aromatic olefinic compounds as used in the present invention include any aromatic olefinic compounds of the general formula (1). 
wherein R is selected from the group consisting of H, CH3 and halogen, and R1 is independently selected from the group consisting of H, OH, an alkyl group having from 1 to 6 carbons, a halogen and xe2x80x94CHxe2x95x90CH2. Preferably, the aromatic olefinic compound is an aromatic vinyl compound including alpha-methylstyrene, p-methylstyrene, alpha-chlorostyrene and divinylbenzene. Most preferred is styrene. Vinylnaphthalene can also be used; it will be appreciated that vinylnaphthalene is not represented by formula 1.
Aliphatic olefinic compounds can also be used, including but not limited to butene, diisobutylene, piperylene, dipentene, isoprene, butadiene, and pinene. A preferred aliphatic olefinic compound is dipentene.
The molar ratio of resorcinol to dicyclopentadiene to olefinic compound is generally as follows. For each mole of resorcinol, at least about 0.25 moles of dicyclopentadiene are used and at least about 0.9 moles of olefinic compound are used. Preferably, the molar ratio of dicyclopentadiene and olefinic compound combined, for each mole of resorcinol, is at least about 1.4. A particularly preferred method utilizes approximately 0.4 moles of dicyclopentadiene and one mole of styrene for every mole of resorcinol.
A catalytically effective amount of catalyst should be used in each of the catalyst addition steps of the present methods, the amount of which can be determined by one skilled in the art. In a preferred embodiment, about 1 gram of PTSA catalyst is used per one mole of resorcinol in the initial charge, with approximately 0.5 grams of catalyst being added at the second charge. The catalyst used in each of the catalyst addition steps is preferably the same, but does not have to be.
One advantage of the method of the present invention is the production of a non-volatile or non-fuming resin having a low free resorcinol content. Other advantages of the procedure are that it generates a low amount of waste since yields are nearly 100%, has minimal toxicity levels, and uses relatively low temperatures and low pressures.
As noted above, the present invention is also directed to the resorcinolic resins produced from the present method. These resorcinolic resins are characterized as being non-volatile or non-fuming, that is, having a low free resorcinol content. Accordingly, these resins can be used in any application in which a non-fuming, low free resorcinol resin is desired.
The resins of the present invention are particularly useful in rubber compounding applications. As discussed above, resorcinol and resorcinol-formaldehyde resins have historically been used in the tire and rubber industry as adhesion promoters for synthetic fabric and steel cord to rubber bonding. Although resorcinol enhances both the mechanical and bonding properties of the cured rubber compounds, the volatility of this material under rubber processing temperatures has prompted some tire manufacturers to use precondensed resorcinol-formaldehyde novolak type resins instead of a resorcinol monomer. The main advantage for using these resins in rubber compound formulations is the reduction of free resorcinol content. The resins of the present invention serve to reduce the free resorcinol content even further. In addition, the resins of the present invention can be produced at a low cost and provide comparable performance for rubber applications than those resins currently commercially available. In addition to their low free resorcinol content, the resins of the present invention, when used in rubber compounding applications, yield a low softening point that enhances the processing of the rubber, provide enhanced adhesion characteristics with reinforcements such as steel, polyester, nylon and others, and provide enhanced mechanical properties such as modulus and elongation.
The present invention is therefore further directed to a vulcanizable rubber composition having improvements in physical and mechanical properties such as dynamic stiffness, hardness, scorch safety and cure time. The vulcanizable rubber composition of the present invention comprises: (a) a rubber component selected from natural rubber, synthetic rubber or combinations thereof; (b) a methylene donor; and (c) a methylene acceptor. The methylene acceptor comprises the low free resorcinol resins of the present invention, such as those prepared by the methods described above.
xe2x80x9cRubberxe2x80x9d as used herein refers to both natural and synthetic rubber. Representative synthetic rubber polymers include the butadiene polymers. Butadiene polymers include those polymers having rubber-like properties, polymerizing butadiene alone or with one or more other polymerizable ethylenically unsaturated compounds, such as styrene, methylstyrene, methyl isopropenyl ketone and acrylonitrile; the butadiene is preferably present in the mixture as at least 40% of the total polymerizable material. Other synthetic rubbers include the neoprene rubbers. Isobutylene rubber (butyl) and ethylene-propylene rubber (EPDM) may also be employed.
Any suitable methylene donor can be used. Preferred are hexamethylenetetraamine (HMTA), di-, tri-, tetra-, penta-, or hexa-N-methylol-melamine or their partially or completely etherified or esterified derivatives, for example hexamethoxymethylmelamine (HMMM), oxazolidine or N-methyl-1,3,5-dioxazine.
Typically, the methylene acceptor is incorporated into the rubber component in an amount ranging from about 1 to 25 parts by weight based on 100 parts by weight of the rubber component (1 to 25 phr). Preferably, the methylene acceptor is incorporated into the rubber component in an amount from about 1 to 5 phr.
Generally, the weight ratio of methylene acceptor to methylene donor is from about 1:10 to 10:1, more preferably 1:3 to 3:1. If HMTA is the methylene donor, the weight ratio is preferably at least about 2:1.
In a preferred embodiment, a vulcanizable rubber composition is provided as described above wherein the methylene acceptor is the addition product of resorcinol and dicyclopentadiene, further reacted with styrene. The preferred embodiment also includes using PTSA as a catalyst both when the addition product of resorcinol and dicyclopentadiene is formed and when further reacting this addition product with styrene.
It will be understood by those skilled in the art that the vulcanizable rubber composition of this invention may also include one or more additives comprising sulfur, carbon black, zinc oxide, silica, an antioxidant, a stearate, an accelerator, an oil or an adhesion promoter.
In another embodiment of this invention, a vulcanizable rubber composition is provided as described above, further comprising (d) a reinforcing material. Any reinforcing material known in the art can be used, including, but not limited to, nylon, rayon, polyester, aramid, glass, steel (brass, zinc or bronze plated) or other organic or inorganic compositions. These reinforcing materials may be in the form of filaments, fibers, cords, or fabrics.
Following the formation of the rubber component, vulcanization can be carried out by methods known in the art.
It will be appreciated that the resin formed by the reaction of the methylene acceptor and methylene donor as described above promotes adhesion between the rubber and the reinforcing materials while simultaneously providing an improvement in the rubber vulcanizate properties such as hardness and dynamic stiffness, as well as improving scorch safety time and providing longer cure times when compared to other compounds. The rubber composition of the present invention further has improved adhesion properties for adhering rubber to the reinforcing materials as described above. Optionally, the reinforcing material can be pretreated or coated with adhesives.